1. Technical Field
The present application relates to an electrochemical energy storing device.
2. Description of the Related Art
In recent years, smartphones, mobile phones, personal digital assistants, laptop computers, camcorders, portable game machines, and the like have gained performance enhancements or long-hour driving abilities, which require the electrochemical energy storing device mounted in any such device to have a higher energy density.
Electrochemical energy storing devices are categorized as electrical double layer capacitors, hybrid capacitors, or batteries.
An electrical double layer capacitor is a device that, by using a pair of polarizable electrodes, allows ions in an electrolyte solution to be adsorbed to both electrodes via charging, thereby forming an electrical double layer, as a result of which electric charge is stored. The current which flows in an electrical double layer capacitor is a non-faradic current. A battery is a device which stores electric charge by allowing oxidation and reduction reactions of an active material to occur at the positive electrode and the negative electrode, the oxidation and reduction reactions being proportionate to the amount of charge that has flowed. A current which flows in a battery is a faradic current. A hybrid capacitor is a device one electrode of which is a polarizable electrode and the other electrode of which is an electrode that causes redox reactions. A hybrid capacitor is also called an electrochemical capacitor.
A big difference between a capacitor/hybrid capacitor and a battery is the changes in ion concentration in the electrolyte solution upon charging and discharging of each device. In a capacitor or a hybrid capacitor, ions in the electrolyte solution are adsorbed to at least one of the electrodes via charging, whereby the ion concentration is reduced. In a battery, during charging or discharging, ions migrate from one electrode to the other electrode, and thus the ion concentration in the electrolyte solution does not change. Therefore, the energy density of a capacitor or a hybrid capacitor depends on the amount of electrolyte solution that is accommodated in the device, whereas a battery has no such dependency. In a battery, a reduced amount of electrolyte solution may be employed while increasing the amounts of positive electrode—and negative electrode—active materials, and thus its energy density can be greater than the energy density of a capacitor or a hybrid capacitor.
Patent Document 1 discloses a hybrid capacitor which oxidizes bromine ions (Br−) or iodine ions (I−) existing in the electrolyte solution to bromine (Br2) or iodine (I2) during charging to thereby store electric charge at the positive electrode, and during discharging reduces them back to the respective ions to thereby release electric charge. Carbon materials such as an activated carbon are used for the positive electrode, whereas ionic liquids such as 1-ethyl-3-methylimidazolium.tetrafluoroborate (abbreviated as EMI.BF4) or organic solvents such as propylene carbonate (abbreviated as PC) are used as the solvent in the electrolyte solution. As the electrolyte salt, 1-ethyl-3-methylimidazolium bromide (abbreviated as EMI.Br), lithium bromide (LiBr), or the like is used, with a preferable concentration of 0.3 to 2.5 mols/L being recited. While potential of the redox reaction between Br− and Br2 is measured with reference to a platinum needle in Japanese Laid-Open Patent Publication No. 2012-119653, their equilibrium potential can also be calculated by using a standard free energy of formation between Br2 and lithium metal (Li), as in Formula (1).3.5 V: Br2+2Li2LiBr  (1)
Japanese Laid-Open Patent Publication No. 2008-141160 discloses a thermally activated capacitor which incorporates an electrolyte that is solid at room temperature. One of them is described to be a capacitor whose electrolyte is an inorganic molten salt containing lithium chloride (LiCl), which is supposedly capable of charging and discharging at the melting point of the molten salt or higher.
Japanese Laid-Open Patent Publication No. 2008-141160 describes that use of an ionic liquid such as 1-ethyl-3-methylimidazolium chloride (abbreviated as EMI.Cl) also enables similar charging and discharging as a capacitor.
Japanese Laid-Open Patent Publication No. 2009-64584 proposes a nonaqueous electrolyte solution secondary battery whose positive electrode contains lithium iodide (LiI), lithium bromide (LiBr), or lithium chloride (LiCl), such that iodine (I2), bromine (Br2), or chlorine (Cl2) is generated via charging and dissolved into the electrolyte solution. It discloses that, at the negative electrode, lithium metal (Li) may deposit or an alloy of Li and aluminum metal (Al) may form during charging, and I2 or the like that is dissolved in the electrolyte solution becomes reduced at the positive electrode during discharging, so that lithium ions (Li+) are released from the negative electrode.